[ QUOTE ]
Intel® Pentium® Extreme Edition Dual Core Processor with HT Technology Cutting edge processing power for intense multi-threaded games and applications.

[/ QUOTE ]

Sounds like a dual core processor (essentially two processors in one) but I have no knowledge of this processor.

[ QUOTE ]
Intel® Pentium® D Processor with Dual Core Technology Accomplish more while running multiple applications, such as editing video while downloading music.

[/ QUOTE ]

Pentium D is basically two Pentium 3 processors in one. Pentium 3 processors run cooler and more efficiently than Pentium 4s, hence their use in dual cores.

[ QUOTE ]
Intel® Pentium® 4 Processor with HT Technology Extreme Edition Advanced performance running multiple high-end games and/or demanding power applications at the same time.

[/ QUOTE ]

Single core Pentium 4 Processor.

Your choice really depends on how you are going to use the computer.

MisterW